1. Field of the Invention
The present invention relates to an oscillator starting control circuit which is suitable for shortening and controlling a starting time of a crystal oscillator circuit and stabilizing an oscillating frequency.
2. Description of the Related Art
In recent years, a crystal oscillating circuit has often been used as a frequency to be a reference in electronic equipment, and furthermore, a transmission data rate has also been increased. In such electronic equipment, an intermittent operation is carried out in order to reduce power consumption. For this reason, there has been desired an oscillating circuit for carrying out starting in a short time for the crystal oscillating circuit used as a frequency to be their reference and outputting a constant oscillating frequency by a low consumed power. Therefore, the crystal oscillating circuit is very useful for a circuit technique.
Moreover, a frequency output from the crystal oscillating circuit is used as a frequency to be a reference in the electronic equipment Therefore, it is necessary to provide an oscillating circuit for carrying out starting in a short time and outputting a constant oscillating frequency after the starting.
Conventionally, the crystal oscillating circuit of this type is constituted as shown in an example of a circuit diagram of FIG. 2 In FIG. 2, 1 denotes an oscillating circuit, 2 denotes an REG voltage circuit, 3 denotes a voltage control circuit, 10 and 11 denote an MOS variable capacity, 12 and 13 denote a fixed capacity, 14 denotes an inverter, 15 denotes a crystal oscillator, 16, 18 and 19 denote a resistor, and 17 denotes a source voltage. The crystal oscillating circuit shown in FIG. 2 has such a structure as to supply a power of the oscillating circuit portion 1 by the REG voltage source 2 and to form an oscillating loop by the crystal oscillator 15, the oscillating circuit portion 1, the MOS capacities 10 and 11, and the fixed capacities 12 and 13, and to output an oscillating waveform.
Referring to the crystal oscillating circuit constituted as described above, an operation thereof will be described. When a source voltage is applied from the source voltage (17), a certain source voltage is supplied from the REG voltage circuit portion (2) to the oscillating circuit portion (1) and the crystal oscillator (15) and an electric potential is applied to the oscillating circuit portion (1) so that an impulse is given to the crystal oscillator (15) Consequently, an oscillating operation is started to be carried out. Then, an oscillating amplitude oscillated in the oscillating circuit portion (1) is output. Moreover, a control voltage is applied from the voltage control circuit (3) to a gate of the MOS variable capacity (10, 11) through the resistor (18, 19) and an MOS capacity value is changed. Consequently, a load capacity CL of the oscillating circuit can be varied to carry out a regulation into a desirable oscillating frequency f0.
An oscillating amplitude and a starting time of the crystal oscillating circuit are determined by a magnitude of a current to be excited to the crystal oscillator (15) when the power supply for the crystal oscillating circuit is turned ON, a negative resistance and a magnitude of the load capacity CL of the crystal oscillating circuit In particular, it is apparent that a starting time is shorter when the load capacity CL is smaller.
When the load capacity CL is reduced, therefore, the starting time is shortened. However, the oscillating frequency f0 is also decreased In the conventional circuit, therefore, the load capacity CL is to be determined from the desirable oscillating frequency f0.
In an oscillating circuit using a variable capacity of a varicap diode, moreover, there has been known a circuit for temporarily changing the load capacity CL and carrying out the starting earlier as in the following JP-A-2001-24435 Publication. With this structure, a voltage to be applied to the varicap diode is limited. For this reason, the load capacity CL cannot be reduced sufficiently so that a sufficient advantage for shortening the starting time cannot be obtained. Moreover, it is impossible to carry out an application to an oscillating circuit using an MOS variable capacity having a high sensitivity. Furthermore, there is a drawback that the starting time is easily varied depending on the starting conditions of a power to be applied to the oscillating circuit.
In the case in which the conventional crystal oscillating circuit shown in FIG. 2 is used or the case in which the crystal oscillating circuit described in the Patent Document 1 is used, thus, an oscillating output in the starting to which a source voltage is applied depends on the load capacity CL which is the cause of a starting time delay. For this reason, there is caused a phenomenon in which a rise time is reduced. When the source voltage rises, moreover, an oscillation is started. Consequently, the starting time of the oscillator also fluctuates depending on the starting time of the source voltage and is not stabilized.
On the other hand, in portable equipment represented by a cell phone, an intermittent operation is usually carried out in order to reduce a consumed power. In order to prolong the lifetime of a battery, it is more advantageous that a duration or the OFF state of the oscillator is longer. For this reason, there is required an oscillator capable of shortening the starting time and stably controlling the starting time.
Moreover, the frequency of the output waveform of the oscillating circuit depends on the capacity value of a variable capacitance unit which is a load capacity of an oscillator. In the case in which a variable capacitance unit having a floating node is used, therefore, the capacity value of the variable capacitance unit is changed with a variation in the voltage of the floating node. After starting the oscillating circuit, therefore, it is impossible to output a certain oscillating frequency.